Laser Crystallization of Amorphous Ge Thin Films via a Nanosecond Pulsed Infrared Laser

Abstract

Understanding the dynamics of the laser crystallization (LC) process of Ge thin films by nanosecond (ns) pulsed infrared (IR) lasers is important for producing homogeneous, crack-free crystalline device-grade films for use in thin-film transistors, photo-detectors, particle detectors, and photovoltaic applications. Our motivation is to describe a ns IR laser-based crystallization process of Ge by implementing suitable parameters to fabricate thin-film devices. Our LC technique was applied to crystallize thin amorphous Ge (a-Ge) films with thicknesses suitable for device applications. The LC process was applied to a 300 nm-thick a-Ge thin film utilizing a 200 ns pulsed IR laser with a wavelength of 1064 nm. Electron-beam-evaporation-deposited a-Ge on glass substrates were subject to successive ns laser pulses with a line focus. The crystallinity of the polycrystalline Ge (pc-Ge) films was evaluated by Raman spectroscopy, optical microscopy, and electron backscatter diffraction (EBSD). LC-Ge exhibited a Raman peak of around 300 cm–1, confirming successful crystallization of a-Ge. pc-Ge domain sizes exceeding several tens of micrometers were observed in EBSD scans. LC of a-Ge minimizes the thermal energy budget of processing and provides flexibility to locally crystallize the film. Our work is the first demonstration of the LC of a-Ge thin films, resulting in domain sizes exceeding tens of micrometers via a ns pulsed IR laser.

Explosive Crystallization Characteristic of Ge–Cu–Te Thin Films for Phase-Change Memory

“…Additionally, the elliptical rather than circular shape of the fanlike EC regions, which is related to EC’s dependence on the temperature gradient of the thin films, can be seen . The EC preferentially stretches longer in the direction of the lowest temperature gradient . At random areas where nucleation begins, crystallization growth is partly sustained, and the neighboring EC growth continues with the accompanying latent heat release .…”

“…Tiny grains result from a high nucleation rate followed by a slow growth rate. Large grains are obtained from a low nucleation rate combined with a high growth rate . The grain size distribution of nucleation-dominated GST phase-change material is mainly composed of 30- and 40-nm grains, whereas growth-dominated Ge–Cu–Te phase-change material shows a larger grain size distribution based on EBSD measurements, with the largest grain size being about 500 nm, as shown in Figure a.…”

“…Large grains are obtained from a low nucleation rate combined with a high growth rate. 28 The grain size distribution of nucleation-dominated GST phase-change material is mainly composed of 30-and 40-nm grains, 29 whereas growthdominated Ge−Cu−Te phase-change material shows a larger grain size distribution based on EBSD measurements, with the largest grain size being about 500 nm, as shown in Figure 3a. Furthermore, the grain size distribution obeys the lognormal distribution, which can be seen from the logarithmic distribution curve of grain size.…”

“…28 At random areas where nucleation begins, crystallization growth is partly sustained, and the neighboring EC growth continues with the accompanying latent heat release. 28 As illustrated in Figure 2c,c', practically complete crystallization is achieved with increasing heat annealing at 200 °C for 30 min. A greater grain size and more continuous microcrystalline network can be obtained by the increased overlapping of radially extending crystal domains.…”

Selective ultrashort laser annealing of amorphous Ge/Si multilayer stacks

“…Recently, germanium thin films with a record-high hole mobility were obtained on flexible plastic using post-growth annealing at relatively low temperature of 500 o C [1]. However, for using inexpensive plastic flexible substrates, it is necessary to reduce their heating to temperatures below 120 o C. Pulsed laser annealing appears to be the only suitable technique for crystallization of amorphous semiconductor films without overheating substrates [2,3].…”